A recording method using an inkjet printer, which is a representative method among various color recording methods, is intended to perform recording by generating small ink droplets and attaching the ink droplets to a variety of record-receiving materials (paper, film, cloth, and the like). In this method, since a recording head is not brought into direct contact with a record-receiving material, less noise is generated and silent recording is achieved. Furthermore, since this method has the feature that it is easy to reduce apparatus size and to increase process speed, the inkjet recording method has been rapidly popularized in recent years, and further growth in the future is expected as well.
Conventionally, aqueous inks prepared by dissolving water-soluble coloring matters in an aqueous medium have been used as inks for fountain pens, felt pens, and the like and as inks for inkjet recording. These aqueous inks generally have water-soluble organic solvents added thereto so that clogging of the ink at pen tips or ink discharge nozzles can be prevented. Further, in regard to these inks, it is required that recorded images with sufficient densities be provided, that clogging at pen tips or nozzles not occur, that the inks have satisfactory dryability on record-receiving materials, that less bleeding occur, that the inks have excellent storage stability, and the like. Furthermore, the water-soluble dyes used therein are required to have high solubility, particularly in water, and to have high solubility in water-soluble organic solvents that are added to the inks. Moreover, the images thus formed are required to have image-fastness properties such as water resistance, light resistance, gas resistance, and moisture resistance.
Among these, gas resistance means resistance to a phenomenon of causing discoloration and fading of a redorded image via an action of ozone gas or the like present in the air and having an oxidizing action on a coloring matter in the record-receiving material. In addition to ozone gas, examples of oxidizing gases having this kind of action include NOx and SOx; however, among these oxidizing gases, ozone gas is regarded as the main causative substance that accelerates the phenomenon of discoloration and fading of inkjet-recorded images. Therefore, resistance to ozone gas in particular tends to be considered the most important. At the surface of a paper for inkjet exclusive use capable of giving photographic-image quality, an ink-receiving layer is provided in order to speed up drying of the ink and to reduce bleeding at high image quality. Regarding the material of this ink-receiving layer, materials such as porous white inorganic substances are frequently used. On such a recording paper, discoloration and fading caused by ozone gas is notably observed. Since this phenomenon of discoloration and fading caused by an oxidizing gas is characteristic of inkjet images, enhancement of ozone-gas resistance is one of the most important problems to be solved in the field of inkjet recording method.
In order to extend the field of application of printing methods using inks in the future, there is strong demand for further enhancements of light resistance, gas resistance, moisture resistance, water resistance, and the like in the ink compositions used in inkjet recording and colored bodies colored by ink compositions.
Inks of various hues have been prepared from various coloring matters, but among them black ink is an important ink that is used in both monochromatic images and full-color images. However, development of a coloring matter that exhibits satisfactory black color, with a hue for which the deep color region and the light color region are neutral, and that has a high print density and lower light source dependency of the hue is technically very difficult. Although tremendous research and development is under way, there are still few coloring matters that exhibit sufficient performance. Accordingly, it has been a general practice to prepare black inks by incorporating a plural number of various coloring matters. However, when an ink is prepared by mixing plural coloring matters, there is a problem in that: 1) the hues may vary depending on the medium (record-receiving material); and 2) discoloration in particular may become significant as a result of the decomposition of coloring matters caused by light or ozone gas, as compared with the case of preparing an ink with a single coloring matter.
An example of a black ink composition for inkjet recording that gives a print material with satisfactory durability in various aspects may be the ink of Patent Document 1. Furthermore, Patent Document 2 has suggested an ink which has further improved ozone resistance and has a satisfactory hue as black color. However, although these inks are ink compositions that have achieved significant improvements even in view of the image-fastness properties of the print materials, there is still demand for further improvement of ozone-gas resistance in particular, and products that sufficiently fulfill market demands are yet to be provided.
Patent Document 1: PCT International Publication No. WO2007/077931
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2009-185133
Patent Document 3: PCT International Publication No. WO2006/001274
Patent Document 4: PCT International Publication No. WO2008/053776
Patent Document 5: Japanese Patent Publication No.
Patent Document 6: German Patent Application, No. 2004488, Specification